Energy is used in most Zynga games. It is used to complete things and level up.
Mafia Wars and Vampire WarsEdit
In these games, it is used to complete missions, which then completes tiers (in Mafia Wars). Each mission has a different amount of energy needed. Doing missions makes you gain experience, which leads to leveling up. Energy is regained by waiting, leveling up, or energy packs.
Energy is used only for digging. Similar to Mafia Wars and Vampire Wars, digging leads to experience, which leads to gaining levels and new stuff.
Energy in FrontierVille is used for just about everything. This includes clearing grass, clobbering or scaring animals, feeding animals,and cutting trees. Like all other games, doing these tasks lead to exp, which leads to gaining levels.